In warehouses with high shelf units special fork-lift trucks have to be used to reach the higher shelves and to move heavier objects. It is easier to place smaller items onto and remove them from the shelves by hand. As the driver of a fork-lift truck often does not have an adequate view of the alleyway from behind the lift and the load there is the danger of collision between the fork-lift truck and persons doing manual order-picking work.
To avoid such accidents it has already been proposed to provide a barrier at the end of each alleyway which, in conjunction with a light barrier and associated electronics, guarantees that only one person or one truck can be in the alleyway at any time. Such barriers greatly impede work, however, because even when the alleyway is empty its opening must be awaited. This is why the personnel often disconnect such barriers so that they can work more quickly. Furthermore, with these known monitoring devices, access to the alleyway is not possible if a person doing manual order-picking work is in the alleyway and a second person, also working manually, wishes to enter the alleyway.
The risk of accidents in alleyways between shelf units could also be remedied in another way by, for example, providing for each fork-lift truck a monitoring device operating on the basis of ultrasound which monitors the space immediately in front of the truck for foreign objects. This possibility does, however, have the disadvantage that many false alarms are generated by other obstacles adjacent to the alleyway. The equipping of the various fork-lift trucks with a monitoring device of this kind also involves high costs.